1. Field of the Invention
The present invention relates to methods and apparatus for enhancing the production of liquid hydrocarbons from subterranean formations penetrated by well bores utilizing one or more sonic energy transducers in the well bores.
2. Description of the Prior Art
The production of liquid hydrocarbons from producing formations and reservoirs is generally assisted to a great extent by naturally occurring forces such as the expanding force of compressed gases, the buoyant and driving force of approaching water and the force of gravity. Primary recovery techniques utilize these forces to cause the liquid hydrocarbons to migrate from subterranean formations into one or more well bores penetrating the formations. Unfortunately, the natural forces are typically only sufficient to allow a small percentage of the total liquid hydrocarbons in formations and reservoirs to be produced.
Secondary recovery techniques are generally employed to recover more of the liquid hydrocarbons in subterranean formations. These techniques utilize extraneous energy forces to supplement the naturally occurring forces in the formations to force the liquid hydrocarbons from the formations into well bores. The extraneous forces can be generated from a large variety of sources including gas injection, steam injection, water injection and the like. These secondary recovery techniques are typically initiated after the primary forces within a formation or reservoir have been at least partially exhausted.
Water flooding is one example of a secondary recovery technique that has been successfully employed in different types of formations. Generally, in accordance with water flooding techniques, one or more injection wells and one or more production wells are utilized. An aqueous solution is injected through the injection wells in order to drive liquid hydrocarbons to the production wells where they are produced. Many modifications to basic water flooding techniques have been developed including the use of certain chemicals and materials in the injection water to help displace the liquid hydrocarbons from the formation. For example, gelling agents are often employed to increase the viscosity of the water and thereby increase its efficiency in driving the oil to the production wells. Surfactants have also been employed to reduce the surface tension of the liquid hydrocarbons and thereby facilitate their production.
Another secondary recovery technique that has been employed to increase the recovery of oil involves the use of low frequency vibration energy. Low frequency vibration from surface or downhole sources has been used to influence liquid hydrocarbon recoveries from subterranean reservoirs. This type of vibration at source-frequencies generally less than about 2000 Hz has been referred to in the literature as sonic, acoustic, seismic, p-wave, or elastic-wave stimulation. For example, stimulation by low frequency vibration has been effectively utilized in some cases in Russia to improve oil production from water flooded reservoirs. Examples from the literature suggest that low frequency stimulation can accelerate or improve ultimate oil recoveries. Explanations for why low frequency stimulation makes a difference vary widely. Examples of such explanations include that the vibration causes the coalescence of oil droplets to reestablish a continuous oil phase, the dislodging of oil droplets so that they can flow as liquid fines, the reduction of capillary forces by altering surface tensions and interfacial tensions and the release of gas which is absorbed at the rock surfaces or dissolved in the water and/or oil phases. For example, U.S. Pat. No. 5,184,678 to Pechkov et al. issued Feb. 9, 1993 discloses a method and apparatus for stimulating fluid production in a producing well utilizing an acoustic energy transducer disposed in the well bore within a producing zone. It is stated in the patent that the acoustic wave radiation transmitted into the producing formation reduces the viscosity of liquid hydrocarbons therein whereby they more readily flow to the well bore.
It is fairly well known that ultrasonic waves can improve and/or accelerate oil production from porous media. The problem with ultrasonic waves is that in general, the depth of penetration or the distance that ultrasonic waves can move into a reservoir from a source is very limited (like less than a few feet), whereas low frequency waves can travel great distances through rock (hundreds to thousands of feet).
While sonic liquid hydrocarbon flow stimulation methods and apparatus have achieved some success in stimulating or enhancing the production of liquid hydrocarbons from subterranean formations, the acoustic energy transducers used have generally lacked sufficient acoustic energy intensity to be significantly effective. Thus, there are continuing needs for improved methods and apparatus which use sonic energy to enhance the production of liquid hydrocarbons such as oil and gas condensate from subterranean formations.